The present invention relates to a four-wheel vehicle drive system for an automotive vehicle having at least front and rear pairs of road wheels. More particularly, the present invention relates to improvements in a transfer gear shift mechanism for shifting from a four-wheel driving condition to a two-wheel driving condition or from a high speed driving condition to a low speed driving condition.
Transfer gear shift mechanisms are disclosed in Japanese Utility Model Application Laid-Open No. Showa 53-21642 and Japanese Patent Application Laid-Open No. Showa 58-211056. These publications disclose transfer gear shift mechanisms which include a high-low speed shift mechanism, a four-two wheel drive shift mechanism, an input shaft, a first output shaft and a second output shaft. The input shaft is rotatably mounted on a transfer casing and is connected to a transmission output shaft of a transmission unit. The high-low speed drive shift mechanism is located between the input shaft and the first output shaft, and the first output shaft is connected to a rear axle shaft.
Accordingly, a vehicle may drive selectively at a high speed or a low speed by shifting the high-low speed drive shift mechanism. The first and second output shafts are rotatably mounted on the transfer casing and the four-two wheel drive shift mechanism is located between the first and second output shafts. The second output shaft is connected to a front axle shaft. Accordingly, the vehicle may drive selectively in a two wheel driving condition or a four wheel driving condition.
Therefore, the vehicle, including the above-mentioned transfer gear shift mechanism, may drive in one of a high speed two wheel driving condition, a high speed four wheel driving condition and a low speed four wheel driving condition.
As disclosed in Japanese Patent Publication No. Showa 57-54333, the high-low speed drive shift mechanism and the four-two wheel drive shift mechanism further include a plurality of engagement sleeves, a pair of fork shafts, a pair of shift forks, an operation rod and a swing arm. The engagement sleeves are slidably mounted on the first output shaft, and the engagement sleeves are selectively positioned at engagement positions or disengagement positions. When one of the engagement sleeves is positioned at the engagement position a torque on the first output shaft is transmitted to the second output shaft, and when the engagement sleeve is positioned at the disengagement position the torque is not transmitted to the second output shaft. The fork shafts are located parallel to the first and second output shafts and the fork shafts are slidably mounted on the transfer casing. Each of the shift forks is secured to a fork shaft, and each of the shift forks engages with an engagement recess of at least one engagement sleeve.
Accordingly, as the fork shaft is moved in the axial direction thereof, the shift fork forces the engagement sleeve to move selectively to the engagement position or the disengagement position. The operation rod is rotatably mounted on the transfer casing and is operated by an operator through an operation lever. The swing arm is rotatably mounted on an end of the operation rod and engages with both fork shafts. When the operation rod rotates, the swing arm forces the fork shafts to move in the axial direction of the fork shafts.
The high-low speed drive shift mechanism and the four-two wheel drive shift mechanism include respective regulators for regulating movements of the fork shafts. Each of the regulators includes a plurality of notches defined on the fork shaft, a locking ball which engages with one of the plurality of notches and which is biased by a spring, and an interlock pin which engages selectively with one of the plurality of notches and which prevents one of the fork shafts from moving with another of the fork shafts.
However, these apparatuses do not have means for preventing disengagement of the engagement sleeves of the high-low speed drive shift mechanism and the two-four wheel drive shift mechanism during rotation of the first output shafts, or means for preventing undue friction between shift fork engagement fingers and engagement sleeve grooves resulting from disengagement prevention.